1. Field of the Invention
The present invention relates to a semiconductor sensor. More particularly, the present invention relates to a semiconductor sensor to detect a physical quantity, such as acceleration, yaw rate, vibration, or the like.
2. Related Arts
Recently, demand has grown for a semiconductor acceleration sensor of greater compactness and lower cost. To this end, a differential-capacitance type semiconductor acceleration sensor employing polycrystalline silicon as an electrode was disclosed in PCT WO 92/03740. A sensor of this type is described utilizing FIGS. 29 and 30. FIG. 29 indicates a plan view of the sensor, and FIG. 30 is an I--I sectional view of FIG. 29.
A movable member 116 having a beam structure is disposed above a silicon substrate 115 with a predetermined gap interposed therebetween. The movable member 116 composed of polycrystalline silicon thin film includes anchor sections 117, 118, 119, and 120, beam sections 121 and 122, a weight section 123, and movable electrode sections 124. The beam sections 121 and 122 extend from the anchor sections 117, 118, 119, and 120 of the movable member 116, and the weight section 123 is supported by these beam sections 121 and 122. The movable electrode sections 124 are formed on weight section 123. Two fixed electrodes 125 are disposed on the silicon substrate 115 to oppose one movable electrode section 124. Accordingly, the structure is such that in a case of acceleration applied in a direction parallel to the surface of the silicon substrate 115 (indicated by Y in FIG. 29), electrostatic capacitance between the movable electrode portion 124 and the fixed electrodes 125 increases on one side and decreases on the other.
In fabricating this sensor, as shown in FIG. 31, a sacrificial layer 126 of silicon oxide film or the like is formed on the silicon substrate 115, and holes 127 are formed in the sacrificial layer 126 at places which become anchor sections. Thereafter, as shown in FIG. 32, a polycrystalline silicon film, which becomes the movable member 116, is deposited on the sacrificial layer 126 and configured in the specified pattern. In continuation, as shown in FIG. 33, the sacrificial layer 126 below the movable member 116 is etched away with an etchant, and the movable member 116 is disposed above the silicon substrate 115 with a predetermined gap interposed therebetween.
However, as is shown in FIG. 34, during film formation, internal stress a is exerted from the interface of the sacrificial layer 126 to the movable member 116 composed of polycrystalline silicon thin film, and internal stress .sigma. gradually changes and increases in the direction of film thickness. As a result of this, an internal stress distribution exists in the direction of film thickness in the movable member 116, causing the movable member to warp. That is to say, as shown in FIG. 29 the movable electrode section 124 assumes a cantilever structure taking the weight section 123 as a fixed end, and the movable electrode section 124 is warped due to the internal stress distribution existing in the direction of film thickness. As a result of this, the movable electrode section 124 and the fixed electrode 125 are not accurately disposed in an opposing relation with good precision. Additionally, deflection due to internal stress distribution is generated in the weight section 123 as well. As a result of this, the movable electrode section 124 protruding from this weight section 123 are also displaced. As a result, the movable electrode section 124 and the fixed electrode 125 are not accurately disposed in an opposing relation with good precision.
As a general means to reduce the internal stress of a film structure body such as this, long-term, high-temperature heat treatment on the film structure body (for example 24 hours at 1,150.degree. C.) is performed. However, this method could not be compounded with an IC process because it would inflict damage on transistors and circuitry provided in the periphery of the movable member 116 on the silicon substrate 115, and as a result, applying peripheral circuitry in this type of semiconductor substrate acceleration sensor was not practical.